It is now known that metal vapors are particularly useful in providing high efficiency and high output power lasers. Metal vapor "collision lasers" generally obtain population inversion through an appropriate relationship between electron collisional excitation and relaxation rates; see, e.g., Gould, Appl. Opt. Suppl. Chem. Lasers, p. 59 (1965). Many metal vapor laser transitions are well known, including copper at 5106A, lead at 7229A, and manganese at 5341A and 12899A. However, of the metal vapor laser transitions, the copper 5106A transition is considered one of the most desirable not only because it is in the visible region, but because it has demonstrated the highest efficiency, peak power, and optical gain of all the metal vapor lasers.
Initially, longitudinal discharge tubes employing stationary mixtures of copper:helium were used to obtain laser action in electrically-pulsed copper discharges with temperatures of approximately between 1500.degree. and 1600.degree.C, see Walter, IEEE J. Quant. Electr. QE-4, 355 (1968). Significant advances in copper lasers have been made by employing flowing copper:helium mixtures in transverse discharge tubes at temperatures up to about 2000.degree.C, see Leonard, Final Report, Airborne Laser Development, Contract No. DAHC 60-70-C-0030 Sept. 1970. The utilization of a transverse discharge permits more effective values of electric field to particle density ratio (E/N) to be obtained which results in substantially higher optical gain and laser energy output per unit volume as well as reduces circuit inductance thus permitting rapid current risetimes.
However, in a metal vapor laser and copper in particular, high temperatures are required to obtain sufficient metal vapor pressures. In both the longitudinal and transverse discharge modes, temperatures in excess of about 1200.degree.C and typically from about 1500.degree.C to 1600.degree.C are required to obtain the preferred copper density of 10.sup.15 atoms/cm.sup.3 for the necessary gain and excitation efficiencies as well as resonance trapping, as discussed hereinafter.
Various attempts have been made to provide the necessary metal vapor densities without high temperatures by utilizing metal-bearing molecular carriers such as, for example, metal carbonyls and other organic complexes. These attempts have been generally unsuccessful because of the large amount of energy lost in dissociation, excitation or ionization of nonlaser species by the electrical discharge, and by the absorption of laser and resonance radiation by these nonlaser species. Furthermore, the organic carriers, once dissociated, fail to provide a continuing vapor pressure since their dissociation is generally irreversible. Thus, the initial density is not maintained nor are the original species. Also, at the high temperatures required, the dissociated organic materials can absorb or scatter visible radiation as well as deposit on the windows thereby degrading the optical qualities of the laser environment. Moreover, solid deposits of dissociated carrier materials can create conducting paths which short circuit the system. Because of the electronegativity of many dissociated carriers, a large number of the electrons can be removed from the discharge through attachment.
Other attempts have been made to employ inelastic collision to create a population inversion and relax the lower transition level by cyclization rather than simultaneous population-depopulation, see U.S. Pat. No. 3,576,500. However, even with cyclization, high temperatures are required for proper metal vapor densities. It was also implied that copper iodide could be used in a cyclic laser to obtain copper atoms at temperatures below 1000.degree.C. However, later attempts to provide lasing by use of metal halides proved unsuccessful. The problems associated with lasing the metal of a metal halide, as opposed to the pure metal, were not apparent, and the suggestions that lower operating temperatures could be used were not realized. See GTE Laboratories Annual Technical Report TR 72-841-1, ONR (Contract NOOO14-11-C-0164) 31 Mar. 1972.
Accordingly, for proper metal vapor lasing conditions, particularly in copper, high temperatures are required by present state of the art means. Practical metal vapor laser systems must be, therefore, constructed of materials capable of withstanding high temperatures.